It has often been a disadvantage with fluorescent lamp electronic ballasts that they have been unable to be dimmed with a standard (phase cut) type of dimmer, particularly in the case of small integral ballast-lamp combinations commonly used in the home to save energy. This is due to the fact that, where there is no power factor correction, the ballast circuit input consists of a rectification stage followed by a large storage capacitor connected to the AC mains supply that provides the DC bus from which the high frequency half bridge and output section is supplied. In such a system, current is drawn from the mains only near the peak of the mains voltage where the storage capacitor charges and not during the remainder of the mains half-cycle.
Virtually all domestic and professional dimming systems are based on triacs. These devices will conduct once they have been fired, only while current flows in excess of the holding current of the deice. These dimmers work very well with a resistive load such as an ordinary Tungsten filament incandescent light bulb as the triac can be fired at any point during the mains half cycle and will continue to conduct until very close to the end of the half-cycle as current is drawn continuously over this period. In this way the lamp current can be adjusted from maximum to zero.
A basic 120 VAC triac based dimmer circuit is shown in FIG. 1. The neutral side is not shown but is connected directly through to the lamp load.
In this conventional triac based dimmer, the triac is triggered once the charge on capacitor C has exceeded the triac threshold. The phase angle in the half cycle when the triac triggers depends on the setting of the potentiometer VR. The longer the RC time constant, the later in the half cycle when the triac triggers. The amount of power supplied to the lamp will determine its brightness. The later in the half cycle that the triac triggers, the lower the brightness level of the lamp.
When a compact fluorescent lamp ballast is connected to a circuit containing such a dimmer, the triac will only conduct if it is fired at a point during the mains half-cycle when the rectified mains voltage is greater than the storage capacitor voltage. In this instance, the capacitor will be charged to the same voltage and the triac will then switch off. In this way it would be possible to adjust the DC bus voltage of the ballast to some extent by adjusting the triac firing point from 90xc2x0 to 180xc2x0. However this will not provide a satisfactory means of controlling the light output. There is also an additional problem encountered due to the fact that a dimmer of this kind requires an inductor in series with the triac to limit the rise time of the current when the device is fired. Without this inductor, mains current harmonics would be produced at frequencies high enough to cause considerable radiated and conducted interference problems. Since the load presented by a ballast circuit is effectively capacitive, when the triac is fired there will be ringing caused by the resonance of the suppression inductor of the dimmer and the capacitive load. This can cause the triac to fire and then switch off as the ringing output voltage swings above and then below the input voltage causing the current to fall below the holding current. This can occur several times during each half cycle, resulting in severe lamp flicker and loss of control of the output.
A system has now been developed, based around, for example, the IR2156 integrated circuit, where the ballast is able to operate with minimal flicker over a considerable portion of the adjustment range of a dimmer and the light output may be controlled over this range from maximum output down to around 10%. In this system, the front end of the ballast has been designed so that when the triac in the dimmer has fired, it will remain on continuously until almost the end of the mains half-cycle. In addition to this there is provided circuitry that detects the firing angle of the triac and adjusts the lamp current by changing the switching frequency hence controlling the light output depending on the level set by the dimmer.
It should be noted that if the dimmer is set too low, the triac will never fire when a capacitive load is connected. Also when the ballast is running and the dimmer is turned too low, there will be insufficient bus voltage for the ballast to be able to operate. Because of these factors it is impossible for the ballast to operate over the complete range of adjustment of the dimmer. There will also be some hysteresis so that when the ballast is being dimmed down and reaches the point where the lamp goes out, the dimmer has to be turned back up some way before the lamp will strike again.